We have previously observed that several aspects of physiological and behavioral development are related to maturational changes in specific populations of brain and/or peripheral monoaminergic neurons. Furthermore, we have recently found that the neuropharmacological, physiological, and behavioral potency of some psychoactive drugs, such as amphetamine, change as a function of the age or the sex of the animal. We now hope to continue our studies examining possible age and sex differences in response to drugs that may alter the physiological and behavioral responses of animals vis a vis their effects on brain monoaminergic neurotransmitters by: 1) more fully characterizing possible maturational changes in neurochemical mechanisms thought to be important for the control of monoaminergic neurotransmitter compounds; 2) determining the possible extent to which maturational changes might alter the interactions among different neurotransmitter systems in selected regions of the brain; 3) more fully characterizing possible maturational changes in the neurochemical, physiological, and behavioral responses of prenatal, neonatal, and adult animals; and 4) continuing studies designed to determine some of the mechanisms that might underlie developmental and sex differences in drug responses.